Sensors based of mechanically resonant elements have long been of interest due to their high sensitivity and electrical readout capability. Resonant sensors allow the use of resonant frequency and/or quality factor (Q) to detect such things as mass, stiffness, and/or damping changes at the resonator surfaces. Applications for such sensors are broad and include, among other things, inertial, pressure, temperature, strain, flow rate, viscosity, density, and chemical/biological sensors.
Many of these sensors are currently constructed from quartz or other piezoelectric materials. Although their performance is proven, they are difficult to miniaturize, integrate, and optimize in a high-volume wafer fabrication process, such as that commonly used to make silicon-based integrated circuits.
A major limitation of resonant sensors exposed to a non-vacuum environment is the performance degradation from viscous damping through various physical mechanisms. The damping directly lowers the Q and therefore the detection limits of the device.
One way that the piezoelectric community has addressed this performance degradation is by constructing crystals with shear (in-plane) vibration modes. Analogous to rowing a boat with the edge of an oar instead of the face, much less force is exerted by the fluid on the mechanical element (and vice versa), and therefore the damping can be minimized. Piezoelectric materials include metal electrodes on two sides for transduction, so placing them in liquids or harsh gaseous environments can require sophisticated passivation or packaging strategies, which can affect both performance and cost. Capacitive transduction, which is favored among non-piezoelectric micromechanical resonators for its simplicity and sensitivity, would also be adversely affected by the presence of fluid or changing environmental conditions. Other transduction methods have their own drawbacks related to cost, performance, interfacing with the sense environment, and/or integration.
There is considerable effort in the MEMS industry to manufacture MEMS resonators (typically silicon) to create oscillators and filters that would compete with conventional quartz devices. However, these devices generally require that all resonator surfaces be packaged in a vacuum with no external environmental exposure. Sealing of these devices can be achieved by capping at low pressures.
Some exemplary efforts to commercialize biosensors based on microscale resonators include piezoelectric devices (e.g., BioScale, Inc. U.S. Pat. No. 07178378, US2006/0196253, Boston Microsystems US2003/0119220, and Intel US2006/0133953) that require exotic materials (compared to silicon) and passivation schemes, and microchannel resonators (e.g., MIT/Affinity Biosensors, US2007/0172940) that require all fluids of interest to flow through a micron-scale resonant channel that provides a natural isolation of the fluid but adds a requirement of microfluidic flow control.
The pressure sensor community demonstrated corrugated diaphragms with isolated capacitive transduction as early as the 1980s (e.g., U.S. Pat. No. 5,750,899, US2003/0183888, U.S. Pat. Nos. 4,809,589, 5,177,579), but these devices move out-of-plane and therefore generally would suffer from overwhelming damping in fluid environments.
Likewise, capacitive micromachined ultrasonic transducers (CMUTs), which resonate in out-of-plane flexural modes, have been used for gas sensing and proposed for liquid sensing (e.g., Stanford University, US2008/0190181) but generally have similar damping limitations.
“In-situ capping” is currently being pursued by several companies to hermetically seal MEMS devices at the wafer scale (e.g., SiTime, in collaboration with Bosch, in an effort to build all-vacuum-encapsulated resonators). Some of the in-situ capping patents in public domain include U.S. Pat. No. 6,635,509 filed by Dalsa Semiconductors, U.S. Pat. No. 5,589,082 filed by University of California Regents, and U.S. Pat. No. 5,937,275 filed by Bosch. Also a European Government sponsored project SUMICAP had some work done on this aspect. However, in the above literature, the main purpose has been to provide a rigid cap to the devices to act as protection.
Resonators have been used for biological/chemical sensing, viscometry, and similar applications. For example, the quartz crystal microbalance (QCM) was demonstrated as a mass sensor in 1959. Since then, it has become a common analytical tool in the chemical and biological sciences for use in vacuum, gas, and liquid environments. Typically, such sensors are relatively large (e.g., centimeters across by hundreds of microns thick), and this relatively large size tends to limit their mass resolution. However, QCM is a pervasive laboratory tool for biosensing, chemical sensing, viscometry, etc.
Silicon-based lateral resonators for liquid applications have been demonstrated utilizing piezoresistive detection without isolating part of the device from the liquid such that the devices may be susceptible to particles that can be trapped under the resonant mass. Robust passivation of piezoresistors is also a significant reliability concern with these devices.
A lateral resonator with exposed electrodes is described in U.S. Pat. No. 7,551,043 filed by Nguyen et al.
An example of prior art targeting frequency reference applications is Nguyen and Xie (U.S. Pat. No. 6,985,051), which utilizes contour modes in silicon with capacitive transduction. In this example, there is no need to isolate any regions of the resonant structure's surface. Similarly, Piazza, Stephanou & Pisano (U.S. Pat. No. 7,492,241), a piezoelectric resonator implementation, is also not designed for partial encapsulation.
Each of the above-referenced patents and published patent applications is hereby incorporated herein by reference in its entirety.